Every year a fresh crop of talented tennis players emerge from the juniors or ITF circuit to challenge the ATP and WTA Tours' best.

Only a handful go on to become superstars.

Who are the tennis stars of tomorrow? What separates one young talent from another?

Identifying up-and-coming players requires an assessment of the past, present and future. What types of players, based on size and style, have succeeded in the past? What type of play works today? Which youngsters playing in the current game have a career trajectory headed upward?

For the purpose of this list we excluded players older than 21 and those who have cracked the Top 20. This includes No. 8 Eugenie Bouchard and No. 6 Milos Raonic. They are no longer up-and-coming. They have already arrived.

Waiting to join or surpass them are a few talented players to keep on your radar.

Coric, 6'1", and still growing, has a big serve and plays as well on clay as he does on hard courts.

American Francis Tiafoe receives more hype than Coric. However, Tiafoe is ranked No. 1151 and lacks Coric's competition experience.

Still babyfaced, Coric defeated 2013 Wimbledon semifinalist Jerzy Janowicz during a Davis Cup match in April. Last month he reached the quarterfinals at a 250-level clay-court tournament in Croatia, where he lost a three-setter to Fabio Fognini.

Jiri Vesely

Jiri Vesely joins an increasing number of basketball-player-sized tennis stars. At 6'6", he's taller than Milos Raonic, but shorter than John Isner.

Oddly, while other up-and-coming stars headed to North America to compete in the U.S. Open series this summer, Vesely opted to stay in Europe and play small clay-court tournaments.

Vesely has a huge upside to his game. In 2013, Vesely was named the ATP Tour's Star of Tomorrow. After a couple of years of building a resume largely on the Challenger Tour, Vesely is improving his play on the ATP Tour.

Belinda Bencic

Pavel Golovkin/Associated PressBelinda Bencic hits a forehand during a match at the 2014 Wimbledon Championships.

Age: 17

Country: Switzerland

WTA Ranking: 58

Belinda Bencic has been called the next Martina Hingis. That's partly because she's Swiss and she's sometimes coached by Hingis' mother.

Bencic reached her first semifinal at the 2014 Family Circle Cup and advanced to the third round at Wimbledon. She finished 2013 ranked outside the Top 200 and has managed to climb to a career-high No. 58.

Bencic and Townsend developed something of a rivalry as juniors players. Last year Bencic defeated Townsend in the girls' championship of the French Open and Wimbledon. With steady improvement from both, they could rekindle that rivalry.

Garbine Muguruza

Ezra Shaw/Getty ImagesGarbine Muguruza hits a forehand during a match at the 2014 Bank of the West Classic in Stanford.

Age: 20

Country: Spain

WTA Ranking: 26

Garbine Muguruza's breakout performance was against her idol Serena Williams at the 2014 French Open. Muguruza upset Williams in the second round, sending the favorite packing and searching for answers.

She went on to reach the quarterfinals, where she lost to eventual champion, Maria Sharapova.

Aggressive, Muguruza, 6', has the power to hang with the biggest hitters. She has also teamed with fellow Spaniard Carla Suarez Navarro in doubles, where she is ranked No. 21.

Jack Sock

Jonathan Moore/Getty ImagesJack Sock hits a forehand during a match at the Western and Southern Open in Cincinnati.

Age: 21

Country: USA

ATP Ranking: 55

After a couple of tough years on the tour, Jack Sock is finally seeing some success. Nicknamed "J. Sizzle," Sock seemed destined to join the ranks of the many disappointing American players, such as Ryan Harrison, whose results have yet to live up to the hype.

Recently, Sock has had more success in doubles than he has in singles. Sock and partner Vasek Pospisil, won the Wimbledon doubles title, shocking the Bryan brothers. The two went 12-0, a record for men's doubles, before losing to the Bryan brothers in the Western and Southern Open final in Cincinnati.

With a rocket serve like Andy Roddick's, Sock is just beginning to play to his potential.

Elina Svitolina

Elsa/Getty ImagesElina Svitolina hits a forehand during the 2014 U.S. Open series.

Age: 19

Country: Ukraine

WTA Ranking: 35

Elina Svitolina upset Wimbledon champion Petra Kvitova at the 2014 Western and Southern Open. It was the biggest win of Svitolina's career and her first against an opponent in the Top 10. She entered the match against Kvitova 0-6 against Top 10 players.

Svitolina won the juniors French Open at age 15 and is the WTA's second-highest ranked teen, behind Madison Keys.

Dominic Thiem

Al Bello/Getty ImagesDominic Thiem prepares to hit a backhand slice during a match at 2014 Wimbledon.

Age: 20

Country: Austria

ATP Ranking: 45

Nicknamed the "Dominator," Dominic Thiem generated buzz with his dash up the rankings. The talented 20-year-old climbed from No. 303 to No. 139 in 2013. Earlier this year, Thiem set a goal to be in the main draw of the Australian Open in 2015.

Although Stephens enjoyed a higher ranking when she was 19, Keys has shown more consistency this year. More importantly, she has a better overall game than Stephens and is far more comfortable at the net.